The invention relates to the general field of metal etching, particularly to etching copper in the presence of nickel-iron, and with particular application to the manufacture of write heads for magnetic disk systems.
Referring to FIG. 1, we show, in schematic representation, a cross-sectional view of a write head for a magnetic disk system. The magnetic field needed to perform the write operation is generated by flat coil 16 made up of a number of turns (typically between about 8 and 18), where 13 is an example of one side of a single turn. Surrounding the flat coil is magnetic material comprising upper and lower pole pieces 12 and 11 respectively. These pole pieces are joined at one end (on the left in this figure) and are separated by small gap 14 at the other end. The magnetic field that is generated by flat coil 16 ends up being concentrated at gap 14. It is sufficiently powerful that the fringing field that extends outwards away from gap 14 is capable of magnetizing the magnetic storage medium over whose surface 15 the head xe2x80x98fliesxe2x80x99. The distance between gap 14 and surface 15 is typically between about 10 and 50 nm.
Clearly, flat coil 16 cannot be in direct contact with lower pole piece 11 so there is always a layer of insulating material (not shown in FIG. 1) between the coil and the lower pole piece. Since the coil is built up using electroplating, it is necessary to provide a conductive layer to act as a seed for the initiation of plating. Once the coil has been grown over this seed layer, the latter must be removed (except where it is directly under the coil) to avoid shorting out the coil.
Two general approaches to the problem of removing the seed layer are practiced in the prior art. The first is sputter etching. This approach is limited by the tendency for some of the sputter etched material to be re-deposited, so some shorting out of the coil remains a possibility. The second approach is to use chemical etching. In principle this overcomes the difficulties associated with sputter etching but this method, too, has associated difficulties. Unfortunately, etchants used to remove copper (notably ammonium persulfate) also attack permalloy (in particular, the nickel component of the permalloy), introducing the possibility of damaging the lower pole piece 11 during seed layer removal. This can happen because there is an ongoing possibility that some of the copper seed layer will be in direct contact with the lower pole piece. Such contact with the copper seed layer is possible because upper pole 12 must contact lower pole 11 at one end so it is convenient to initially plate it on the exposed area of lower pole 11. It is also necessary to electroform a copper stud near the device in order to make electrical wiring connection after the upper pole has been completed. Since the seed layer covers the whole wafer, there is direct contact of the copper seed layer on top of upper pole 12 which should be removed after stud plating. The present invention provides a solution to this problem.
A routine search of the prior art was performed but no processes similar to the exact process of the present invention were found. Several references of interest were, however, encountered. For example, U.S. Pat. No. 5,304,284 (Jagannathan et al.) shows a process for etching copper in the presence of more reactive metals. The method depends on the use of non-aqueous solutions and is based on creating a suitable balance between solutes of different oxidation potentials. One embodiment uses alkenes and alkynes as complexing agents.
In U.S. Pat. No. 5,800,726, Cotte et al. address the inverse problem to that solved by the present invention. i.e. it is the copper that is to be protected while other metals are being etched. Heim et al. (U.S. Pat. No. 5,935,644) show a pole and coil plating process while in U.S. Pat. No. 5,639,509 Schemmel teaches a process for forming a magnetic data transducer.
It has been an object of the present invention to provide a process for manufacturing a write coil for use in the write head of a magnetic disk system.
Another object of the invention has been to use electroplating on a seed layer and to subsequently remove said seed layer without affecting other materials that are close by, notably nickel-iron.
A further object of the invention has been that removal of the seed layer be accomplished by means of wet etching.
These objects have been achieved by removing the seed layer through etching in a solution of ammonium persulfate to which has been added the complexing agent 1,4,8,11 tetraazundecane. This suppresses the reduction of Cu++ to Cu, thereby increasing the dissolution rate of copper while decreasing that of nickel-iron. Two ways of implementing this are describedxe2x80x94adding the complexing agent directly to the ammonium persulfate and introducing the 1,4,8,11 tetraazundecane through a dipping process that precedes conventional etching in the ammonium persulfate.